1. Field of the Invention
This invention relates to a process for toning a colored electrophotographic photosensitive layer after a developing treatment.
2. Description of the Prior Art
It is known that an electrophotographic photosensitive layer can be produced by dispersing a photoconductive powdered material into a resinous binder, and for this purpose various photoconductive materials are already known to be usuable such as cadmium sulfide, zinc oxide, zinc sulfide etc. Though the color of the electrophotographic photosensitive layer is not an important factor in electrophotographic processes containing the step of transferring a toner image onto an ordinary paper sheet, the color significantly affects the image final quality obtained if the process does not contain such transfer step. In the case of using such a photoconductive powdered material, however, spectral sensitizing dyes are frequently added in order to correct the spectral sensitivity thereof or to improve the sensitivity thereof.
Consequently, it has been necessary to prepare a photosensitive layer making a compromise between the final image quality and the spectral sensitivity of such layer. To spectrally sensitize photoconductive powdered materials various dyes are known, e.g., the xanthene dyes, triphenyl methane dyes, azo dyes, cyanine dyes, merocyanine dyes, etc.
Such spectral sensitizing dyes show fading at different rates when an electrophotographic photosensitive layer containing the same is exposed to light. Consequently, if the color of a photosensitive layer is selected to compromise with spectral sensitivity directly after developing, discoloration or fading of the photosensitive layer after a period of time may result in a disagreeable color. However, in the case of storing an unused electrophotographic photosensitive layer in a dark place, it is possible to obtain a fairly long shelf-like even when such elements are sensitized with cyanine dyes, which generally show very poor stability against light. This long shelf life often leads to a careless selection of sensitizing dyes without giving consideration to the light fastness thereof, and it is often impossible to maintain final image quality at a constant level for a prolonged period. In order to prevent this drawback it has been proposed to remove the sensitizing dye from the electrophotographic photosensitive layer after the developing treatment thereof, such as by a bleaching bath consisting of a solution of an organic acid (U.S. Pat. No. 3,250,614). The composition of such a bleaching bath, however, must leave the photosensitive layer and the image intact, and therefore severe restrictions are placed upon the bleaching bath, depending upon the pigment used as the toner and the resin component employed in the developer.